This invention is directed to middle ear implants and more particularly to a novel middle ear prosthesis that responds to the stapedial reflex.
The middle ear transmits sound vibrations from the ear drum or tympanic membrane to the inner ear via auditory ossicles or movable bones that are linked together and known as the malleus, the incus and the stapes. The malleus, which is the outermost bone, is embedded in the tympanic membrane and connected to the incus, which is the middle bone. The stapes, which is the innermost bone, is connected to the incus and attached to the cochlea or inner ear at the margin of an oval window of the cochlea.
Optimum transmission of sound from the outer ear to the inner ear requires effective contact between the malleus and the tympanic membrane to sense sound induced vibrations of the tympanic membrane, functional movement of the malleus, the incus and the stapes in response to the vibrations of the tympanic membrane, and effective contact between the stapes and the cochlea to transmit sound vibrations to the inner ear.
Diseases of the middle ear such as otosclerosis or congenital ear defects can fix the stapes. The stapes is usually removed and replaced by a piston and often by a piece of vein graft in order to restore desirable vibration of the inner ear fluids. The impedence transfer and acoustic impedence of the annular ligament are the two main biophysical parameters to restore.
While much attention has been directed to functional replacement of the auditory ossicles, very little has been done to restore the function of the stapedial tendon which is normally attached to the stapes. The stapedial tendon is usually severed or detached when the stapes superstructure is removed or degenerated from disease. I have found that the stapedial reflex, which causes contraction of the stapedial tendon and consequential movement of the stapes footplate, plays a significant role in speech recognition.
The stapedial tendon is known to have a variable response to noise depending upon sound intensity, much like the iris has a variable response to different light intensities. The stapedial tendon retracts under loud sound impingement to reduce the amount of movement transmitted to the oval window of the cochlea by the stapes. Retraction of the stapedial tendon also raises the resonant frequency of the auditory ossicles and attenuates sound. The stapedial reflex thus helps guard against acoustic trauma to the ear.
My findings indicate that the stapedial reflex function is instrumental in the area of sound discrimination when several different people are speaking. Through the action of the stapedial tendon it is possible that high frequency tones are more easily discriminated and voices can be distinguished or enhanced in a noisy setting.
Detachment of the stapedial tendon from the stapes upon surgical removal of the stapes superstructure prevents retraction of the stapes footplate by the stapedial tendon. The beneficial effects of the stapedial reflex in influencing stapes footplate movement are thus lost.
It is thus desirable to provide a middle ear prosthesis that permits restoration of the stapedial reflex function to influence movement of the stapes footplate and selectively change the resonant frequency of nonremoved auditory ossicles.